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ME6405 Timers Timers Mati Chessin 10/07/04 The George W. Woodruff School of Mechanical Engineering ME6405 Timers Contents • Intro to Timers • General Uses • 555 Timer The George W. Woodruff School of Mechanical Engineering ME6405 Timers Introduction What is a timer? • A timer is a device that oscillates at an exact, programmable frequency • In an electronic device, the timer will be used to coordinate actions and keep components working at the same rate The George W. Woodruff School of Mechanical Engineering ME6405 Timers Introduction • 1880 - Piezoelectric effect discovered by Jacques and Pierre Curie • 1905 - First hydrothermal growth of quartz in a laboratory by G. Spezia • 1918 - First use of piezoelectric crystal in an oscillator • 1927 - First quartz crystal clock built • 1934 - First practical temp. compensated cut, the AT-cut, developed • 1956 - First commercially grown cultured quartz available • 1971 - Introduction of 555 timer by Signetics (later acquired by Philips) The George W. Woodruff School of Mechanical Engineering ME6405 Timers Introduction How do timers work? • Timers are regulated by an oscillating device, usually a crystal • The crystal is made of a piezoelectric material, such as quartz • As crystal vibrates, it produces voltage when compressed and the opposite voltage when expanded • By vibrating the crystal at its natural frequency, a circuit can be created to produce a very regular timing signal • Higher oscillation frequencies yield lower output voltages • A purely electronic timer can be made by utilizing a capacitor and resistor to regularly store and discharge voltage The George W. Woodruff School of Mechanical Engineering ME6405 Timers General Uses • Timers are used in many electronic devices: – Examples: Clocks, radar, computers, microwaves, robots, HC11, etc… – Used to measure the time between inputs (leading or falling edge of signal) – Used for pulse width measurement – Used to create a time delay The George W. Woodruff School of Mechanical Engineering ME6405 Timers General Uses A Single device can have multiple timers. These could be both built-in or external. For example: HC11 – Uses crystal for timing HC11 could regulate a 555 timer that controls an LED The George W. Woodruff School of Mechanical Engineering ME6405 Timers General Uses Input signal Common Applications: • Monostable Multivibrator – Holds a state (“high” or “low”) until triggered – Once triggered, keeps other state for a determined amount of time High Low Dt The George W. Woodruff School of Mechanical Engineering ME6405 Timers General Uses Common Applications: • Bistable Multivibrator – Can hold either the high or low state – When triggered, it switches state Input signal Low The George W. Woodruff School of Mechanical Engineering ME6405 Timers General Uses High Common Applications: • Astable Multivibrator – Automatically switches between high and low states – Becomes a rectangular wave generator Low Dtlow Dthigh High Low Dtlow Dthigh The George W. Woodruff School of Mechanical Engineering ME6405 Timers The 555 Timer • Most common timer • Costs about $0.50 • A single pulse lasts 1.1 ms • 15,400 seconds is the longest time it can be set to The George W. Woodruff School of Mechanical Engineering ME6405 Timers The 555 Timer Astable 555 + pin 2 pin 7 0 3/2 Vcc 0 control F/F pin 3 + 0 +Vcc RESET 0 + 0 The George W. Woodruff School of Mechanical Engineering ME6405 Timers The 555 Timer The George W. Woodruff School of Mechanical Engineering ME6405 Timers References • http://www.me.gatech.edu/mechatronics_la b/LabMaterials/EExercise2.pdf • http://www.me.gatech.edu/charles.ume/me6 405Fall01/ClassNotes/Timer_fall_01.ppt • http://www.madsci.org/posts/archives/may9 8/895289339.Eg.r.html The George W. Woodruff School of Mechanical Engineering